1. Field of the Invention
The present invention relates generally to bearings, and more specifically to a squeeze film damper for a bearing in which the damper has variable damping.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A turbomachine, such as a turbopump, utilizes damper seals and rigidly mounted rolling element bearings to dampen and rotatably support the shaft. Prior Art turbo pumps use an interstage bearing and damper seals to provide support and damping to the rotor. Damper seals provide positive stiffness but require leakage flow to operate. Squeeze film dampers (SFD) are inherently stable and do not produce and destabilizing forces. One problem with Prior Art squeeze film dampers is that they have no stiffness and therefore cannot support the rotor. The Prior Art configurations require a centering spring so that the squeeze film damper does not bottom out on one side or whirl about the outer diameter. This centering spring controls the overall support stiffness until the squeeze film damper has bottomed out on the housing. When the squeeze film damper and centering spring have bottomed out, the support has minimal damping and a very high stiffness from the housing. Some prior art squeeze film dampers also have air entrainment or cavitations. This air entrainment or cavitations limit the performance of the squeeze film damper. It is this combination of high and low stiffness with variable damping that make it difficult to predict the performance of the squeeze film damper.
Rolling element bearings have been used on many prior art turbopump designs. Their reliability and consistency during startup is important for many applications from main engines to upper stage engines that need the ability to have multiple start and stop cycles. Rolling element bearings are also important for turbo pumps that are designed to have long time periods between overhauls because of their consistency and predictable wear. Rolling element bearings are also excellent for deep throttling since they do not rely on hydrostatic pressure to support the rotor. It is for these reasons and many others that future turbo machines will continue to use rolling element bearings as the primary rotor support.
One weakness of rolling element bearings is the lack of damping they provide to the rotor. A system without adequate damping will transmit high forces through the bearings causing premature failure of the bearings and other high cycle fatigue (HCF) limited components. In most cases, the HCF can be mitigated by a change in design, usually causing the system mass and complexity to increase. In addition, a lightly damped rotor can exhibit large amplitude vibration and rotor dynamic instability. Rotor dynamic instability causes extreme forces in the rotor and casing and can limit component life or in extreme cases can be destructive. Therefore, most Prior Art turbo pumps use an interstage seal called a damper seal that is configured to minimize leakage and provide damping to the system. Unfortunately, a damper seal requires a minimum pressure and flow rate to be effective, therefore putting a lower limit on the pressure and mass flow through the secondary flow circuit. Any leakage through the secondary flow system causes a large performance penalty for a turbopump, so having a minimum allowable flow rate through the turbopump is not ideal. Another drawback of a damper seal is that while it does provide good damping characteristics, it also produces a cross-coupled stiffness which is a destabilizing force. The Prior Art damper seal has proven successful, however many drawbacks exist that provide limitations not only to the bearings, but also to the entire turbomachine.
One prior art patent, U.S. Pat. No. 5,603,574 issued to Ide et al on Feb. 18, 1997 and entitled FLUID DAMPENED SUPPORT HAVING VARIABLE STIFNESS AND DAMPING shows a fluid dampened bearing with a flexible fluid dampened membrane that is used to support the bearing, and an incompressible fluid is provided in the spaces to allow fluid dampening. However, the fluid damper of the Ide et al invention does not change the stiffness of the damper as in the present invention.
U.S. Pat. No. 5,080,499 issued to Klusman et al on Hanuary 14, 1992 entitled HIGH TEMPERATURE SQUEEZE FILM DAMPER shows a squeeze film damper with a pair of metal bellows straddling annular exposed channels, the metal bellows being welded to the fixed and floating journals, in which the bellows confines the damping fluid. The bellows in the Klusman invention is not pressurized by a pressure source from outside the bellows, and the pressure acting within the bellows cannot be varied or regulated in order to control the stiffness of the squeeze film damper as in the present invention. The bellows in the Klusman invention act more like a reservoir for the damping fluid in the squeeze film damper. The bellows of the applicant's invention is more than a reservoir.
In this applicant's previously filed and co-pending application to the Squeeze Film Damper with Variable Support Stiffness, the flexible bellows was formed to flex outward from the center to produce the variable stiffness for the damper. Since the previous application was filed, the applicant has discovered through modeling that forming the flexible bellows in the reverse sense in that the bellows flexes inward instead of outward, the stiffness of the damper can vary significantly more than the outwardly flexing bellows damper. The inward flexing bellows can vary stiffness up to about 300% to 400% from the base stiffness of the damper. The base stiffness is the flexible bellows of the present invention without any pressure applied to either chamber 33 or 42.
It is an object of the present invention to provide for a squeeze film damper for a rolling element bearing that will provide a variable spring force to center and support the bearing and squeeze film damper independent of the operating speed.
Another object of the present invention is to provide for a squeeze film damper for a rolling element bearing in which the squeeze film damper fluid is separated from the secondary fluid flow circuit.
Still another object of the present invention is to provide for a squeeze film damper for a rolling element bearing that requires less axial length than the prior art damper seal and roller element bearing.
Yet still another object of the present invention is to provide for a squeeze film damper for a rolling element bearing in which the spring rate of the damper can be changed rapidly and accurately.
These and other objects of the present invention will become apparent with the description of the present invention below.